1. Field of the Invention
The present invention relates generally to a terminal for telegraph and telephone system, and more particularly, to a terminal for telegraph and telephone systems using a positive temperature coefficient thermistor element (hereinafter referred to as a PTC element) as an overvoltage/overcurrent protecting component.
2. Description of the Prior Art
Examples of terminals for telegraph and telephone systems includes a telephone set, a facsimile, a PBX (private branch exchange) and the like used on the subscriber's side. Many of the terminals contain a bell circuit and a speaking circuit. More specifically, a bell circuit 1 and a speaking circuit 2 are connected to subscriber's lines 4a and 4b through hook-switches 3a and 3b, as shown in FIG. 2. Reference numeral 5 denotes a surge absorbing element, which is constituted by, for example, a varistor and is connected so as to absorb a surge current.
The hook-switches 3a and 3b are connected to the bell circuit 1, as shown in FIG. 2, at the on-hook time. At the off-hook time, the hook-switches 3a and 3b are switched so as to be connected to the speaking circuit 2. A voltage applied to the circuits in an interface portion 6 is generally 48 volts. When the terminal starts reception in the connected state shown in FIG. 2, that is, the on-hook state, a bell voltage such as an AC voltage of 75 volts (in the case of Japan) or an AC voltage of 150 volts (in the case of U.S.) is applied, so that a bell begins to ring. When a user takes up a receiver to bring the terminal into the off-hook state, the hook-switches 3a and 3b are switched so as to be connected to the speaking circuit 2. Accordingly, the supply of the voltage to the bell circuit 1 is cut off, so that the bell stops ringing. Consequently, the speaking circuit 2 is connected to the subscriber's lines 4a and 4b, so that the terminal enters the speaking state.
However, in the interface portion 6 in the above described terminal, a very large overvoltage may, in some cases, be applied due to a failure, an interconnection error or the like of the terminal. For example, the above described bell voltage may be erroneously applied to the speaking circuit 2 due to a failure, or the interface portion 6 may be erroneously connected to a commercial power supply due to an interconnection error, so that an overvoltage of approximately 200 volts may, in some cases, be applied. In order to provide protection against such an overvoltage, therefore, an overvoltage protecting component as shown in FIGS. 3 and 4 has conventionally been connected to the interface portion 6.
More specifically, not only the surge absorbing element 5 but also a current fuse 7 serving as an overvoltage protecting component is connected to the interface portion 6 comprising the bell circuit and the speaking circuit, as in the construction shown in FIG. 3. Alternatively, a PTC element 8 serving as an overvoltage protecting component is connected to the interface portion 6, as in the construction shown in FIG. 4.
In the construction shown in FIG. 3, when an overvoltage/overcurrent is applied, the current fuse 7 is fused, so that the interface portion 6 in the terminal is protected. Similarly, in the construction shown in FIG. 4, the interface portion 6 is protected by the current limiting function of the PTC element 8.
In recent years, a protecting operation against a very large overvoltage of 600 volts has been required for safety reasons for the terminal for telegraph and telephone systems. The reason for this is to provide protection against a case where a high-voltage line is brought into erroneous contact with a telephone line as a result of a tornado or an earthquake.
In the terminal using the current fuse 7 as an overvoltage/overcurrent protecting component, as shown in FIG. 3, when the terminal is erroneously connected to the commercial power supply or the like due to an interconnection error, the current fuse 7 is fused, to protect the interface portion 6. In addition, when a large overvoltage of 600 volts is applied as described above, the current fuse 7 is also fused, to reliably protect the interface portion 6. Consequently, a requirement of UL1459, which is a standard related to a telegraph and telephone apparatus is satisfied, thereby making it possible to reliably protect the terminal for telegraph and telephone systems.
However, the current fuse 7 used as an overvoltage/overcurrent protecting component has the disadvantage of having no restoring characteristics for the protecting operation. More specifically, every time the current fuse 7 is fused, the current fuse 7 must be replaced with a new current fuse. Consequently, complicated maintenance work must be performed.
On the other hand, in the terminal using the PTC element 8 shown in FIG. 4, the PTC element 8 is an overvoltage/overcurrent protecting component having restoring characteristics for the protecting operation. Accordingly, the above described complicated maintenance work can be omitted. However, the overvoltage/overcurrent protecting component using the conventional PTC element can only protect the interface portion 6 against the erroneous application of the bell voltage and the erroneous connection of the terminal to the commercial power supply of approximately 200 volts. It cannot reliably protect the interface portion 6 against a vary large overvoltage of 600 volts. More specifically, when a very large overvoltage of 600 volts is applied, the PTC element 8 may, in some cases, be short-circuited and destroyed, so that a very large current is applied to the speaking circuit, causing a serious accident such as ignition of the terminal. Consequently, the terminal using the conventional PTC element does not satisfy the requirement of a standard for requiring protection against an overvoltage of 600 volts, for example, a standard of UL1459, CSA or Bellcore.